The present invention relates to a method of production of molds for injection molding, particularly of tools for injection molding of plastics.
Known methods of production of tools for injection molding of plastics include the utilization of high-grade tools which produce molds from fine-steel blocks by material removing operations. The quality and accuracy of the injection molding workpieces depend primarily on the hardness of the workpiece material. The material-removing working is correspondingly tedious and cost-consuming, so that in the event of complicated molds for production of tools high investment is needed which is possible only for high numbers to be produced. When the production of such tools is planned, it is necessary to be sure in advance that the workpiece to be produced has a design which is optimal for the sale and the use. Changes in the tools are almost not possible or can be done in a small range, for example with the aid of electroerosion, which is also a tedious and expensive process. Changes of a greater scale can be accomplished only by production of a new tool.
A tool of a less hard material is less expensive to manufacture; however, it makes possible a lower number of injection and produces workpieces of lower accuracy. In the sense of costs, such less expensive tools are profitable for objects which do not have exact mass and whose number are relatively small, such as for example toys. Such simple tools for correspondingly simple workpieces are cast for example of zinc alloys. They are also formed as hard nickel mold parts galvanoplastically from a primary mold core. The tools cast on zinc basis have the disadvantage that zinc has a tendency to bubble formation. These simple tools can withstand at best 5000-10,000 manufacturing cycles, whereas fine-steel tools of hard-grade alloyed glass-hard steel can carry out 1,000,000 injections.
When before the production of high-grade tools the workpiece is tested in such a manner that, first, they are produced in a testing process with simpler tools, additional costs are involved for testing tools, on the one hand, and there is a risk that the research will be of no value because of the insufficient and unreliable accuracy, on the other hand. The high cost of production of high-grade injection tools in accordance with the existing methods considerably blocks the expansion of the injection molding technique. Each production of a new model involves a high investment risk. In the case when a model is to be introduced into a market, high tooling costs play a negative tool. As soon as the market requires changes, new investments must be made.
These conditions are typical both for the synthetic plastics industry and especially for the tire industry which is in a phase of continuous innovation, since improved profiles for winter tires are searched for as a replacement for spoked tires. During test research the profile is often cut from a smooth protector, since the production of small series in accordance with injection-molding methods is very expensive and for one individual tire four or more segment-shaped tools are needed to be assembled on a common tool. Changes in molds required for special tires, such as tires for street and ground construction, for farming tractors and for cross-country vehicles involve, in relation to the number of pieces, too high costs.
Similar cost difficulties are encountered in the case of injection molding of metal, so here the tools are not so expensive, but they are not durable.